Which of these numbers is composite? ${19,\ 41,\ 45,\ 61,\ 79}$
Explanation: A composite number is a number that has more than two factors (including 1 and itself). 19, 41, 61, and 79 each have only two factors. The factors of 45 are 1, 3, 5, 9, 15, and 45. Thus, 45 is the composite number.